Segfault.

Double think about this part:

   while(temp < size)
    {
        temp = temp+i;

Let's assume: size = 4, temp =3 , i =2
If it's still not clear, add cout to the part where you build res.

68 warriors have solved it already in C++. The issue is in your solution most probably and not in my kata.
Maybe there is i.e. an additional character not visible in your final string at the end.
I don't know your solution and I am not able to help you with it. Investigate your code again. You can post it here marked as a spoiler. Maybe somebody can investigate it for you.